The Origin and Formation of Clay Minerals in Soils: Past, Present and Future Perspectives

Clay Minerals (1999) 34, 7–25 The origin and formation of clay minerals in soils: past, present and future perspectives M. J. WILSON Macaulay Land Use Research Institute, Craigiebuckler, Aberdeen AB15 8QH, UK (Received 23 September 1997; revised 15 January 1998) ABSTRACT: The origin and formation of soil clay minerals, namely micas, vermiculites, smectites, chlorites and interlayered minerals, interstratified minerals and kaolin minerals, are broadly reviewed in the context of research over the past half century. In particular, the pioneer overviews of Millot, Pedro and Duchaufour in France and of Jackson in the USA, are considered in the light of selected examples from the huge volume of work that has since taken place on this topic. It is concluded that these early overviews may still be regarded as being generally valid, although it may be that too much emphasis has been placed upon transformation mechanisms and not enough upon neoformation processes. This review also highlights some of the many problems pertaining to the origin and formation of soil clays that remain to be resolved. It has long been recognized that the minerals in the detail remained to be filled in, as well as a time that clay (<2 mm) fractions of soils play a crucial role in immediately pre-dated the widespread utilization in determining their major physical and chemical soil science of analytical techniques such as properties, and inevitably, questions concerning scanning electron microscopy, electron probe the origin and formation of these minerals have microanalysis, Mo¨ssbauer spectroscopy, electron assumed some prominence in soil science research. spin resonance spectroscopy and infrared spectro- This review considers some important aspects of scopy. This review will therefore attempt to these questions and is confined to clay minerals in summarize the general conclusions that had been soils as they are generally understood, that is as a arrived at regarding the origin and formation of medium for plant growth. certain clay minerals in soils in the early 1960s, the The occurrence of clay minerals in saprolites or developments since that time and the situation as it weathered rock is not discussed to any great extent currently stands, and to consider the outstanding because many studies have shown that saprolites are problems to be addressed in the future. In this characterized by physicochemical conditions that context, particular attention will be paid to micas, are different to those in soils. Intimate grain-to- vermiculites, smectites, chlorites, interstratified grain contacts promote a special chemical environ- minerals and kaolin minerals. As in all such ment on a local scale, bringing about the formation reviews, a certain amount of subjectivity and of transient clay mineral phases which quickly personal preference is perhaps inevitable. disappear in the overlying soil. For the purposes of this review the perspective of PAST PERSPECTIVES the past will be taken to be that of the early 1960s, which is a convenient historical benchmark in that The reviews of Millot (1965) and Jackson (1964) it represents a time when the overall structural represent a convenient starting point with regard to characteristics of the layer silicate clay minerals the origin and formation of soil clays as assessed were to a large extent known, even though much principally by the X-ray diffraction and microscopic # 1999 The Mineralogical Society 8 M. J. Wilson techniques that were available at the time. Millot drained, acid and base-depleted tropical environ- distinguished three principal processes to account ments, where an abundant supply of water ensures for the genesis of clay minerals, which may occur the required silica and alumina. Montmorillonite is at different points in the geochemical cycle more typical of a poorly drained or hydromorphic including weathering or soil formation at the soils under alkaline conditions, rich in Mg and Ca earth’s surface. These processes are: (a) detrital ions and where Si, Al and commonly Fe tend to inheritance whereby, for soils, clay minerals are accumulate. inherited from pre-existing parent rock or weathered These generalizations were further developed and materials; (b) transformation where the essential synthesized by French soil scientists, most notably silicate structure of the clay mineral is maintained Pedro and Duchaufour. Pedro (1964) distinguished to a large extent, but with major change in the two mineral weathering processes implicated in the interlayer region of the structure; and (c) neoforma- formation of clay minerals in soils which he termed tion, where the clay mineral forms through crystal- ‘hydrolysis’ and ‘acidolysis’. (A summary of this lization of gels or solutions. work in English is given in Pedro (1982)). Inherited soil clays may be of an extremely Hydrolysis of minerals occurs through dilute diverse and complex nature, reflecting both the solutions in the pH range 5À9.6 and may be total variety of the parent rock as well as the or partial. Total hydrolysis leads to the removal of transformation and neoformation processes that all elements including silica, and to the precipita- may have occurred in previous weathering environ- tion of gibbsite and kaolinite minerals, whereas ments. In order to understand current clay mineral- partial hydrolysis, under different conditions, leads forming processes in a soil, it is essential that the to the formation of smectite minerals (Pedro, 1982). contribution of inheritance is clearly understood. Acidolysis operates when the soil solution pH is <5 With regard to transformation of clay minerals, a or has strongly complexing properties and can again general example would be: be total or limited. Total acidolysis involves complete solubilization of minerals with no illite ? vermiculite ? smectite. precipitation of Al. Limited acidolysis leads to the This reaction proceeds through a process of fixation of Al in octahedral and interlamellar depletion and exchange of interlayer K and positions in layer silicates. In general, acidolysis concomitant decrease of layer charge. Such is associated with podzols, podzolic brown soils and changes are, however, deceptively simple and acid brown soils of cold temperate climates, have given rise to much debate about the precise whereas hydrolysis is dominant in ferrallitic soils mechanisms involved, as will be discussed later. of the warm humid tropics and is prominent in Millot distinguished ‘degradation’ and ‘aggradation’ warm temperate zones and in the dry subtropics as separate forms of the transformation process. The (Table 1). above conversion of illite to montmorillonite A similar synthesis relating to soil clays and involves depletion of elements from illite and is weathering was described by Duchaufour (1960) termed degradation, but the reverse reaction who distinguished geochemical weathering under (aggradation) involves addition of K and other near neutral conditions with no organic acid anions elements. Millot considered that degradation was and typical of tropical environments, and biochem- characteristic of weathering rocks and soils, but that ical weathering under acid conditions with organic aggradation was rare in such environments. anions and typical of temperate climates. The The formation of clay minerals through neofor- former is characterized by neoformed clay minerals mation clearly depends upon the appropriate and the latter by clay minerals formed by physicochemical conditions of the immediate transformation. weathering environment, such as the pH, composi- The overview of Jackson (1964) concerning the tion and concentration of the soil solutions, as well distribution, stability and weathering reactions of as the nature of the starting material and factors clay minerals in soils is, largely, consistent with the relating to the external environment like tempera- conclusions of the French soil scientists. Jackson ture, rainfall and percolation rate. Millot described described the dominant clay mineral types occur- kaolinite and montmorillonite as classical products ring in the Soil Orders of the taxonomy of the of neoformation in soils forming under contrasting USDA Soil Survey Staff (1960). There was a conditions. Thus, kaolinite is typical of freely predominance of micaceous, interstratified or Origin and formation of clay minerals in soils 9 TABLE 1. Occurrence of soil clay minerals in relation to weathering process, principal mechanism and soil types (Adapted from Pedro, 1982). Weathering Principal Clay Principal process mechanism minerals soil types Transformation Acidolysis Smectites Spodosols (partial) Vermiculites Inceptisols Al-intergrades Entisols Al-chlorite Neoformation Hydrolysis Smectites Spodosols (partial) Mollisols Alfisols Hydrolysis Kaolinite Ultisols (total) Halloysite Oxisols Gibbsite interlayered layer silicate minerals in soils such as 2:1?1:1 pedogeochemical reactions. A major role Entisols, Inceptisols and Spodosols which occur for precursor crystals was also suggested in the widely in temperate climates. Clay minerals in soils growth of montmorillonite with hydroxy units of of tropical climates such as Oxisols and Ultisols, Al, Fe and Mg forming on pre-existing mont- were dominated by kaolinite and halloysite, in morillonite crystallites, which become silicated to addition to gibbsite and sesquioxides. However, the form new montmorillonite layers. Jackson (1964) occurrence of 2:1 to 2:2 intergrades was also concluded that ‘‘the almost universal occurrence of indicated in both

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